1. Field of the Invention
The present invention relates to a semiconductor device including a capacitor element of an MIM structure and, particularly, to a semiconductor device such as an SRAM composed of a plurality of memories cells.
2. Description of Related Art
A CMOS (Complementary Metal Oxide Semiconductor) type SRAM (Static Random Access Memory), which is a semiconductor device, is composed of a plurality of memory cells arranged in an array. A memory cell 100 includes two pMOS-FETs (Field Effect Transistors) 101 and 102 and four nMOS-FETs 103 to 106 as shown in FIG. 5.
The nMOS-FETs 105 and 106 are switching transistors for making an access to a particular bit cell from a peripheral circuit such as a sense amplifier. The nMOS-FETs 103 and 104 and the pMOS-FETs 101 and 102 form a part that actually stores information.
A portion to connect the gates of a pair of nMOS-FETs and a pMOS-FET with the drains of another pair of nMOS-FETs and a pMOS-FET is called a node or a node line. It is also called cross coupling because they appear to cross each other in a circuit diagram.
If a radiation is incident on the memory cell 100 of an SRAM, an electron-hole pair (e-h pair) is generated as shown in FIG. 6. Because electrons move to the n-layer and holes move to the p-layer, a reverse-biased PN junction is temporarily short-circuited, so that charges flow from a node electrode into a depletion layer. As a result, the charges in a storage node are lost and the information stored in the memory cell 100 is inverted. This is called soft error.
In order to avoid the soft error, in a memory cell 110 shown in FIG. 7, capacitor elements 111 and 112 having larger capacitance than a charge which is generated due to a ray are added to the nodes to thereby prevent data breakdown in the electron-hole pair generated due to the ray (see Japanese Unexamined Patent Application Publication No. 2005-183420, for example).
In the SRAM 120 which is disclosed in Japanese Unexamined Patent Application Publication No. 2005-183420, a metal line used as a node line serves also as a lower electrode 122 of a capacitor element 121 as shown in FIGS. 8 and 9. Specifically, a capacitor film 123 and an upper electrode 124 are stacked on top of the lower electrode 122, which is the node line, thereby forming the capacitor element 121 of the MIM (Metal-Insulator-Metal) structure. The capacitor element 121 is added to the node of the memory cell.
A method of manufacturing the capacitor element 121 having the above-described structure is briefly described hereinafter with reference to FIGS. 10A to 10D. Referring first to FIG. 10A, the lower electrode 122, which is made of tungsten or the like, is formed to project outward from the end faces of the upper electrode 124 and the capacitor film 123.
Referring next to FIG. 10B, a capacitor material 123a, which is a capacitor insulating layer such as Ta2O5, is deposited on top of the lower electrode 122. Referring then to FIG. 10C, an electrode material 124a, which is made of titanium nitride or the like, is deposited on top of the capacitor material 123a. Referring finally to FIG. 10D, the capacitor material 123a and the electrode material 124a are etched to form the upper electrode 124 and the capacitor film 123.
Another example of the capacitor element having the MIM structure is such that a barrier layer, which is an insulating layer containing silicon and oxygen, is formed between an upper electrode and a capacitor film and between the capacitor film and a lower electrode (see Japanese Unexamined Patent Application Publication No. 2004-266010, for example).
This structure not only adds capacitance but also prevents oxygen deficiencies in the capacitor film which occurs at the interface between a dielectric and an electrode in order to avoid an increase in leakage current, capacitor element's temperature dependence and electric field dependence.
Still another example of the capacitor element having the MIM structure is such that an upper electrode and a capacitor film have a common end face and an interlayer film is formed between the capacitor film and a lower electrode so as to extend to the position at a certain distance inward from the end face (see Japanese Unexamined Patent Application Publication Nos. 2005-019831, 02-310958 and 02-144964, for example).
It is demanded for a recent SRAM to have small cells. Therefore, a VCC line, which is one of a power supply electrode, a GND line, which is the other one of the power supply electrode, and a node electrode are placed in close proximity.
In an SRAM, VCC lines and GND lines are formed in the mesh structure or the like in an upper layer of the device. If a power supply electrode of an SRAM doubles as a lower electrode of a capacitor element as described above, it is necessary to connect the lower electrode to the VDD line or the GND line placed thereabove.
Therefore, the upper electrode has an opening hole, so that the lower electrode of the capacitor element is connected with the power supply line placed thereabove by a vertical electrode which is placed in the opening hole. In such a case, the lower electrode is formed across the opening hole.
In the process of manufacturing the capacitor element having such a structure, the capacitor material 123a and the electrode material 124a, which are deposited sequentially on top of the lower electrode 122, are etched as described above with reference to FIGS. 10A to 10D. However, damage can occur in the capacitor film in the process of creating the opening hole of the upper electrode and the capacitor film by etching.
Further, because the end face of the upper electrode, which also serves as the inner side face of the opening hole, and the lower electrode cross each other in the above-described structure, an electric field concentration occurs there. If there is damage in the capacitor film where the electric field is concentrated, it can cause reduction of dielectric strength or the occurrence of leakage current in the capacitor element.
Furthermore, if an opening hole is created in the upper electrode and the capacitor film by etching, it can cause damage to occur in the lower electrode located at the opening hole, which deteriorates the characteristics. In the capacitor element disclosed in Japanese Unexamined Patent Application Publication No. 2005-019831 and so on, the lower electrode extends to the outside of the end face of the upper electrode and the capacitor film. Therefore, damage can occur in the lower electrode when etching the upper electrode and the capacitor film.
Japanese Unexamined Patent Application Publication No. 02-310958 discloses a capacitor element of the stacked structure. In this capacitor element, an insulating film is placed immediately below the end of an upper electrode and a capacitor film. In this technique, however, the capacitor film and a lower electrode are merely in contact with each other at an opening of the insulating film.
Further, in this capacitor element, an upper electrode is composed of two layers, and a natural silicon oxide film is placed between the capacitor film and the lower electrode. Therefore, the capacitor element of Japanese Unexamined Patent Application Publication No. 02-310958 needs to have the four-layer structure composed of the upper electrode, the capacitor film, the natural silicon oxide film, and the lower electrode. Because the natural silicon oxide film is placed between the capacitor film and the lower electrode, the capacitance of the capacitor element decreases in this structure.